The present invention generally concerns a negative feedback control circuit having a common line for input and output signals, and more particularly, a television receiver tuner having a local oscillator which is controlled by a phase-locked loop.
Television receivers include a tuner section for heterodyning received signals, e.g., broadcast or cable RF signals, to produce an intermediate frequency (IF) signal. The IF signal is processed by the IF signal section and the picture and sound components are further processed by respective signal processing sections to produce a video picture signal for displaying an image on a CRT, and to produce an audio signal for producing an audio response by a loudspeaker.
The tuner section includes at least one controlled local oscillator and at least one RF amplifier wherein both have voltage controlled tuning circuits controlled in response to the magnitude of a tuning voltage (TV), which is generated in the exemplary embodiment by the PLL responsive to the selected channel. Typically the tuned circuits include combinations of inductors and varactor diodes which are reverse biased so as to exhibit a variable capacitive reactance having a value determined by the magnitude of the tuning voltage.
Phase locked loop (PLL) tuning systems employ a closed loop negative feedback arrangement for controlling the frequency of the local oscillator signal. The closed loop arrangement includes programmable digital counters as programmable frequency dividers by which a signal is generated which is the frequency to the local oscillator signal divided by a programmable factor related to the selected channel. The programmable frequency divider can be preceded by an optional prescaler circuit which frequency divides the incoming signal by a constant factor. As used herein, statements referring to frequency division, prescaler, and frequency dividers are independent of whether or not the frequency division is programmable.
The frequency and phase of the frequency divided signals are compared by a phase detector to a reference frequency signal to derive a control voltage for controlling the frequency of the controlled local oscillator. For a tuning system employing a PLL, tuning voltage (TV) for tuning the RF amplifiers is also derived from the PLL control voltage developed in response to the channel selected.
The output line from the local oscillator provides an input signal for the PLL. The return output line from the PLL provides the input control signal for the controllable local oscillator. These two lines go from proximally spaced terminals at the PLL to another pair of proximally spaced terminals at the local oscillator. These lines carry high frequency signals which can radiate, and carry pick-up sensitive tuning voltage signals. It is desirable to minimize these adverse effects. Such minimization can be achieved by reducing the length of the lines and/or reducing the number of lines. In particular, reducing the number of lines has the advantage of simplifying printed circuit board layout and reducing the pick-up and radiation produced by printed circuit wiring leads.